Semiconductor manufacturers are interested in taking measurements at selected points on a wafer from the center out to the edges thereof. In order to obtain accurate measurement, the focal spot of the probe beam must not extend beyond the wafer edge. As can be appreciated, any portion of the beam which extends beyond the wafer's edge will not be reflected resulting in an unexpected reduction in measured intensity which leads to errors in the analysis. In addition, edge effects can cause scattering, also reducing the accuracy of the measurement.
Because of this problem, the metrology device must be configured to limit how close the center of the focal spot can be moved to the edge during a measurement. This restriction is not typically a problem if the probe beam spot is relatively small and circular. However, when the probe beam is large and has an elliptical shape, problems arise. This problem is present with current X-ray reflection measurements and ellipsometry systems, where a focused X-ray beam is directed onto a wafer at a non-normal angle of incidence. For example, U.S. Pat. No. 5,619,548 and PCT WO 01/71325, both incorporated herein by reference describe methods and apparatus for X-ray reflectometry with a focused X-ray beam directed onto a wafer with an angle of incidence in a range between 87.8 and 89.9 degrees from normal. In another example, U.S. Pat. No. 5,973,787, also incorporated herein by reference, describes an ellipsometry system with an angle of incidence of an optical beam in the range between 30 and 70 degrees from normal.
As an example, FIG. 1 illustrates a probe beam 2 directed onto a wafer 1 at a high angle of incidence Φ as measured from the normal 12. Although the probe beam itself has a generally circular cross section, the spot 7 on the wafer surface is elliptical, having a short axis W substantially corresponding to the beam diameter and a long axis L which is dependent upon, among other factors, the angle of incidence of the beam on the sample. In particular, for a circular beam, the long axis of the elliptical beam spot is defined byL=d/cos Φ  [1]where d is the beam diameter and Φ is the angle of incidence of the beam. As can be appreciated from equation (1), increasing the angle of incidence of the beam increase the long axis of the ellipse. For a high angle of incidence, such as used in an X-Ray reflectometer, the long axis of the ellipse can be more than five times longer than the short axis. This elliptical beam spot has caused problems when attempting to take measurements close to the edge of a wafer due to the scattering and edge effects mentioned above.
This problem will be discussed with reference to FIG. 2. More specifically, in many existing systems, the wafer rests on a movable stage. The stage is used to vary the position of the wafer with respect to the beam spot. For ease of explanation, FIG. 2 shows an X-Y coordinate system with the center of the wafer being at Ox, Oy. If it is desired to measure at a site near the top center of wafer (Ox, Ny), the stage is moved so the beam spot is located at position “A”. As can be seen, the center of the beam spot can be moved quite close to the wafer edge without the edges of the beam extending beyond the wafer edge.
In contrast, if it desired to measure at position (Nx, Oy), which is a similar distance from the wafer's edge as position A, the beam spot would extend at over the edge of the wafer. Accordingly, the center of the beam spot must be moved to position B, which is farther from the edge than position A. Accessing measurement areas on a wafer using only X, Y linear motions to position an elliptical beam spot means that the area actually accessible for measurement is elliptical in configuration as indicated by ellipse 8 of FIG. 2. (It should be understood that the illustrated dimensions are not drawn to scale, particularly the beam spot which has been greatly enlarged.)